Abstract

This document specifies XML digital signature processing rules and syntax. XML
Signatures provide integrity, message authentication, and/or signer authentication
services for data of any type, whether located within the XML that includes the signature
or elsewhere.

Status of this document

This specification is a last call Working Draft of the IETF/W3C XML Signature Working Group. The Working Group
invites review from the IETF community, W3C members, and other interested parties. This
last call serves as a statement that the Working Group believes that the specification
satisfies the relevant terms of the charter and requirements document. The W3C last
call ends March 27, 2000; the IETF last call should substantially overlap but may not
exactly coincide with this period. Subsequently, the Working Group plans to issue a
specification that addresses any comments resulting from the review and propose it as a
W3C Candidate Recommendation and IETF Proposed Standard.

This document continues to be a draft document and may be updated,
replaced, or obsoleted by other documents at any time. While the Working Group feels the
design meets our requirements we especially welcome comments on the following topics:
security concerns, URI/IDREF usage, XPath, DTD/schema specification, and implementation
experience. Please send comments to the editors and cc: the list <w3c-ietf-xmldsig@w3.org>. Publication as a
Working Draft does not imply endorsement by the W3C membership or IESG. It is
inappropriate to cite W3C Drafts as other than "work in progress." A list of
current W3C working drafts can be found at http://www.w3.org/TR.
Current IETF drafts can be found at http://www.ietf.org/1id-abstracts.html.

This document specifies XML syntax and processing rules for creating and representing
digital signatures. XML Signatures can be applied to any digital content (data object), including XML. An XML Signature may be
applied to the content of one or more resources. Enveloped or enveloping
signatures are over data within the same XML document as the signature; detached signatures are over
data external to the signature document.

This specification also defines other useful types including methods of referencing
collections of resources, algorithms, and keying information and management.

For readability, brevity, and historic reasons this document uses the term
"signature" to generally refer to to digital authentication values of all
types.Obviously, the term is also stricly used to refer to authentication values that are
based on public keys and that provide signer authentication. When specifically discussing
authentication values based on symmetric secret key codes we use the terms authenticators
or authentication codes. (See section 8.3:Check the Security Model.

The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this specification
are to be interpreted as described in RFC2119
[KEYWORDS]:

"they MUST only be used where it is actually required for interoperation or to
limit behavior which has potential for causing harm (e.g., limiting retransmissions)"

Consequently, we use these capitalized keywords to unambiguously specify requirements
over protocol and application features and behavior that affect the interoperability and
security of implementations. These key words are not used (capitalized) to describe XML
grammar; schema definitions unambiguously describe such requirements and we wish to
reserve the prominence of these terms for the natural language descriptions of protocols
and features. For instance, an XML attribute might be described as being
"optional." Compliance with the XML-namespace specification is described as
"REQUIRED."

No provision is made for an explicit version number in this syntax. If a future version
is needed, it will use a different namespace The XML namespace [XML-ns] URI that MUST be used by implementations of this (dated)
specification is:

This namespace is also used as the prefix for algorithm identifiers used by this
specification. While applications MUST support XML and XML-namespaces, the use of internal entities [XML] or our "dsig" XML namespace prefix
and defaulting/scoping conventions are OPTIONAL; we use these facilities to provide
compact and readable examples.

This specification uses Uniform Resource Identifiers [URI] to identify resources,
algorithms, and semantics. The URI in the namespace declaration above is also used as a
prefix for URIs under the control of this specification. For resources not under the
control of this specification, we use the designated Uniform Resource Names [URN] or
Uniform Resource Locators [URL] defined by its normative external specification. If an
external specification has not allocated itself a Uniform Resource Identifier we allocate
an identifier under our own namespace. For instance:

SignatureProperties is identified and
defined by this specification's namespace

http://www.w3.org/2000/02/xmldsig#SignatureProperties

XSLT is identified and defined by an external
namespace

http://www.w3.org/TR/1999/PR-xslt-19991008

SHA1 is identified via this specification's namespace
and defined via a normative reference

In this section, an informal representation and examples are used to describe the
structure of the XML signature syntax. This representation and examples may omit
attributes, details and potential features that are fully explained later.

XML Signatures are be applied to arbitrary digital
content (data objects) via an indirection. Data objects are digested, the resulting
value is placed in an element (with other information) and that element is then digested
and cryptographically signed. XML digital signatures are represented by the Signature
element which has the following structure (where "?" denotes zero or one
occurrence; "+" denotes one or more occurrences; and "*" denotes zero
or more occurrences):

The content that is signed was, at the time of signature creation, referred to as an
identified resource to which the specified
transforms were applied. Within an XML document, signatures are related to data objects
via IDREFs [XML] and the data can be included within an enveloping signature or can enclose
an enveloped signature. Signatures
are related to external data objects via URIs [URI] and the
signature and data object are detached.

[s02-16] The required SignedInfo element is the information
that is actually signed. Core validation
of SignedInfo consists of two mandatory processes: validation of the signature over SignedInfo
and validation of each Reference
digest within SignedInfo. Note that the algorithms used in calculating the SignatureValue
are also included in the signed information while the SignatureValue element
is outside SignedInfo.

[s03-05] The CanonicalizationMethod is the algorithm that is
used to canonicalize the SignedInfo element before it is digested as part of
the signature operation. In the absence of a CanonicalizationMethod element,
no canonicalization is done.

[s06-07] The SignatureMethod is the algorithm that is used to
convert the canonicalized SignedInfo into the SignatureValue. It
is a combination of a digest algorithm and a key dependent algorithm and possibly other
algorithms such as padding, for example RSA-SHA1. The algorithm names are signed to resist
attacks based on substituting a weaker algorithm. To promote application interoperability
we specify mandatory to implement signature algorithms. We specify additional algorithms
as recommended or optional and the signature design does permit arbitrary user algorithm
specification.

[s08-15] Each Reference element includes the digest method
and resulting digest value calculated over the identified data object. It also may include
transformations that produced the input to the digest operation. A data object is signed
by computing its digest value and a signature over that value. The signature is later
checked via reference and signature validation.

[s18-20]KeyInfo indicates the key to be used to validate the
signature. Possible forms for identification include certificates, key names, and key
agreement algorithms and information -- we define only a few. KeyInfo is OPTIONAL for two
reasons. First, the signer may not wish to reveal key information to all signature
verifiers. Second, the information may be known within the application's context and need
not be represented explicitly. Since KeyInfo is outside of SignedInfo,
if the signer wishes to bind the keying information to the signature, a Reference
can easily identify and include the KeyInfo as part of the signature.

[s08] The optional URI attribute of Reference identifies
the data object to be signed. This attribute may be omitted on at most one Reference
in a Signature. (This limitation is imposed in order to ensure that references
and objects may be matched unambiguously.)

[s08-11] This identification, along with the transforms, is a description
provided by the signer on how they obtained the signed data object in the form it was
digested (i.e. the digested content). The verifier may obtain the digested content in
another method so long as the digest verifies. In particular, the verifier may obtain the
content from a different location such as a local store) than that specified in the URI.

[s09-11] Transforms is an optional ordered list of processing steps that
were applied to the resource's content before it was digested. Transforms can include
operations such as canonicalization, encoding/decoding (including compression/inflation),
XSLT and XPath. XPath transforms permit the signer to derive an XML document that omits
portions of the source document. Consequently those excluded portions can change without
affecting signature validity. For example, if the resource being signed encloses the
signature itself, such a transform must be used to exclude the signature value from its
own computation. If no Transforms element is present, the resource's content
is digested directly. While we specify mandatory (and optional) canonicalization and
decoding algorithms, user specified transforms are permitted.

[s12-14] DigestMethod is the algorithm applied to the data after Transforms
is applied (if specified) to yield the DigestValue. The signing of the DigestValue
is what bind's a resources content to the signer's key.

This specification does not address mechanisms for making statements or assertions.
Instead, this document defines what it means for something to be signed by an XML
Signature (message authentication, integrity, and/or signer authentication). Applications
that wish to represent other semantics must rely upon other technologies, such as [XML, XML-schema, RDF].
However, we do define a SignatureProperties element type for the inclusion of
assertions about the signature itself (e.g., signature semantics, the time of signing or
the serial number of hardware used in cryptographic processes). Such assertions may be
signed by including a Reference for the SignatureProperties in SignedInfo.
While the signing application should be very careful about what it signs (it should
understand what is in the SignatureProperty) a receiving application has no obligation to
understand that semantic (though its parent trust engine may wish to).

Any content about the signature generation may be located within the SignatureProperty
element. The mandatory Target attribute references the Signature
element to which the property applies.

Consider the preceding example with an additional reference to a local Object
that includes a SignatureProperty element. (Such a signature would not only
be detached[p01] but enveloping[p03].)

[p04] The optional Type attribute provides information about the
resource identified by the URI. In particular, it can indicate that it is an Object,
SignatureProperties, or Manifest element. This can be used by
applications to initiate special processing of some Reference elements.
References to an XML data element within an Object element SHOULD identify
the actual element pointed to. Where the element content is not XML (perhaps it is binary
or encoded data) the reference should identify the Object and the ReferenceType, if given, SHOULD indicate Object. Note that Type
is advisory and no action based on it or checking of its correctness is required by core
behavior.

[p11]Object is an optional element for including data
objects within the signature element or elsewhere. The Object can be
optionally typed and/or encoded.

[p12] Signature properties, such as time of signing, can be optionally
signed by identifying them from within a Reference. (These properties are
traditionally called signature "attributes" although that term has no
relationship to the XML term "attribute".)

The Manifest element is provided to meet additional requirements not
directly addressed by the mandatory parts of this specification. Two requirements and the
way the Manifest satisfies them follows.

First, applications frequently need to efficiently sign multiple data objects even
where the signature operation itself is an expensive public key signature. This
requirement can be met by including multiple Reference elements within SignedInfo
since the inclusion of each digest secures the data digested. However, some applications
may not want the core validation
behavior associated with this approach because it requires every Reference
within SignedInfo to undergo reference validation -- the DigestValue elements are
checked. These applications may wish to reserve reference validation decision logic to
themselves. For example, an application might receive a signature validSignedInfo element that includes three Reference
elements. If a single Reference fails (the identified data object when
digested does not yield the specified DigestValue) the signature would fail core validation. However, the application
may wish to treat the signature over the two valid Reference elements as
valid or take different actions depending on which fails. To accomplish this, SignedInfo
would reference a Manifest element that contains one or more Reference
elements (with the same structure as those in SignedInfo). Then, reference
validation of the Manifest is under application control.

Second, consider an application where many signatures (using different keys) are
applied to a large number of documents. An inefficient solution is to have a separate
signature (per key) repeatedly applied to a large SignedInfo element (with
many References); this is wasteful and redundant. A more efficient solution
is to include many references in a single Manifest that is then referenced from
multiple Signature elements.

The example below includes a Reference that signs a Manifest
found within the Object element.

Note, there may be valid signatures that some signature applications are unable to
validate. Reasons for this include failure to implement optional parts of this
specification, inability or unwillingness to execute specified algorithms, or inability or
unwillingness to dereference specified URIs (some URI schemes may cause undesireable side
affects), etc.

3.2.1 Reference Validation

For each Reference in SignedInfo:

Obtain the data object to be digested. (The signature application may rely upon the
identification (URI) and Transforms provided by the signer in
the Reference element, or it may obtain the content through other means such
as a local cache.)

Digest the resulting data object using the DigestMethod specified in its Reference
specification.

Compare the generated digest value against DigestValue in SignedInfo;
if there is any mismatch, validation fails.

3.2.2 Signature Validation

Canonicalize the SignedInfo element based on the CanonicalizationMethod,
if any, in SignedInfo.

Obtain the keying information from KeyInfo or from an external source.

Use the specified SignatureMethod to validate the SignatureValue
over the (optionally canonicalized) SignedInfo element.

The general structure of an XML signature is described in section 2: Signature Overview. This section provides detailed syntax of the
core signature features and actual examples. Features described in this section are
mandatory to implement unless otherwise indicated. The syntax is defined via DTDs and [XML-Schema] with the following XML preamble, declaration, and
internal entity:

The SignatureValue element contains the actual value of the digital
signature; it is encoded according by the identifier specified in SignatureMethod.
Base64 [MIME] is the encoding method for all SignatureMethods
specified within this specification. While we specify a mandatory (and optional) SignatureMethod
algorithm, user specified algorithms (with their own encodings) are permitted.

The structure of SignedInfo includes the canonicalization algorithm (if
any), a signature algorithm, and one or more references. The SignedInfo
element may contain an optional ID attribute that will allow it to be referenced by other
signatures and objects.

SignedInfo does not include explicit signature or digest properties (such
as calculation time, cryptographic device serial number, etc.). If an application needs to
associate properties with the signature or digest, it may include such information in a SignatureProperties
element within an Object element.

CanonicalizationMethod is an optional element that specifies the canonicalization
algorithm applied to the SignedInfo element prior to performing signature
calculations. This element uses the general structure for algorithms described in section
6.1: Algorithm Identifiers. Options include a minimal algorithm
(CRLF and charset normalization) and more extensive operations such as [XML-C14N]. If the CanonicalizationMethod is omitted,
no change is made to SignedInfo before digesting. (Note this may lead to
interoperability failures as other applications may not serialize it as the creators
application did by default. See section 7.)

SignatureMethod is a required element that specifies the algorithm used for
signature generation and validation. This algorithm identifies all cryptographic functions
involved in the signature operation (e.g. hashing, public key algorithms, MACs, padding,
etc.). This element uses the general structure here for algorithms described in section
6.1. While there is a single identifier, that identifier may specify a format
containing multiple distinct signature values.

Reference is an element that may occur one or more times. It specifies a
digest algorithm and digest value, and optionally the object being signed, the type of the
object, and/or a list of transforms to be applied prior to digesting. The identification
and transforms describe how the digested content (i.e., the input to the digest method)
was created. The type attribute facilitates the processing of referenced
data. For example, while this specification makes no requirements over external data, an
application may wish to signal that the referent is a Manifest. An optional ID
attribute permits a Reference to be referenced from elsewhere.

The URI attribute identifies a data object using a URI-Reference [URI], as
specified by RFC2396 [URI]. Note that a null URI (URI="") is permitted
and identifies the XML document that the reference is contained within (the root element).
XML Signature applications MUST be able to parse URI syntax. We RECOMMEND they be able to
dereference null URIs and URIs in the HTTP scheme. (See the section
3.2.1:Reference Validation for a further comment on URI dereferencing.)

[URI] permits identifiers that specify a fragment identifier via a separating pound
symbol '#'. (The meaning of the fragment is defined by the resource's MIME type). XML
Signature applications MUST support the the XPointer 'bare name' [Xptr]
shortcut after '#' so as to identify IDs within XML documents. The results are serialized
as specified in section 6.6.3:XPath Filtering. For example,

Identifies the element with ID attribute value 'chapter1' of the XML resource containing
the signature.

Otherwise, support of other fragment/MIME types (e.g., PDF) or XML addressing
mechanisms (e.g., [XPath, Xptr]) is
OPTIONAL, though we RECOMMEND support of [XPath]. Regardless,
such fragment identification and addressing SHOULD be given under Transforms (not
as part of the URI) so that they can be fully identified and specified. For instance, one
could reference a fragment of a document that is encoded by using the ReferenceURI
to identify the resource, and one Transform to specify decoding, and a second to
specify an XPath selection.

If the URI attribute is omitted all-together, the receiving application is
expected to know the identity of the object. For example, a lightweight data protocol
might omit this attribute given the identity of the object is part of the application
context. This attribute may be omitted from at most one Reference in any
particular SignedInfo, or Manifest.

The digest algorithm is applied to the data octets being secured. Typically that is
done by locating (possibly using the URI if provided) the data and transforming
it. If the data is an XML document, the document is assumed to be unparsed prior to the
application of Transforms. If there are no Transforms, then the data is
passed to the digest algorithm unmodified.

The optional Type attribute contains information about the type of object
being signed. This is represented as a URI. For example:

The Type attribute applies to the item being pointed at, not its contents. For example,
a reference that identifies an Object element containing a SignatureProperties
element is still of type #Object. The type attribute is advisory. No
validation of the type information is required by this specification.

The optional Transforms element contains an ordered list of Transform
elements; these describe how the signer obtained the data object that was digested. The
output of each Transform (octets) serves as input to the next Transform.
The input to the first Transform is the source data. The output from the last
Transform is the input for the DigestMethod algorithm. When
transforms are applied the signer is not signing the native (original) document but the
resulting (transformed) document [section 8.2: Only What is
"Seen" Should be Signed].

Each Transform consists of an Algorithm attribute, optional MimeType
and Charset attributes, and content parameters, if any, appropriate for the
given algorithm. The Algorithm attribute value specifies the name of the
algorithm to be performed, and the Transform content provides additional data
to govern the algorithm's processing of the input resource (see section 6.1: Algorithm Identifiers and Implementation Requirements).

The optional MimeType and Charset (IANA registered character
set) attributes are made available to algorithms which need and are otherwise unable to
deduce that information about the data they are processing.

Examples of transforms include but are not limited to base-64 decoding [MIME], canonicalization [XML-c14n], XPath
filtering [XPath], and XSLT [XSLT]. The
generic definition of the Transform element also allows application-specific
transform algorithms. For example, the transform could be a decompression routine given by
a Java class appearing as a base-64 encoded parameter to a Java Transform
algorithm. However, applications should refrain from using application-specific transforms
if they wish their signatures to be verifiable outside of their application domain.
Section 6.6: Transform Algorithms defines the list of
standard transformations.

DigestMethod is a required element that identifies the digest algorithm to be applied
to the signed object. This element uses the general structure here for algorithms
specified in section 6.1: Algorithm Identifiers.

KeyInfo may contain keys, names, certificates and other public key management
information (such as in-band key distribution or agreement data or data supporting any
other method.) This specification defines a few simple types but applications may place
their own key identification and exchange semantics within this element through the
XML-namespace facility. [XML-ns]

KeyInfo is an optional element that enables the recipient(s) to obtain the
key(s) needed to validate the signature. If omitted, the recipient is expected to be able
to identify the key based on application context information. Multiple declarations within
KeyInfo refer to the same key. Applications may define and use any mechanism they choose
through inclusion of elements from a different namespace.

KeyName contains an identifier for the key, which may be useful to the
recipient. It may be a simple string name, index, encoded DN, email address, etc.

KeyValue contains the actual key(s) used to validate the signature. If the
key is sent in protected form, the MgmtData element should be used. Specific
types must be defined for each algorithm type (see algorithms).

RetrievalMethod is a URI (including optional query parameters) that may be
used to obtain key and/or certificate information.

X509Data contains an identifier of the key/cert used for validation (either
an IssuerSerial value, a subject name, or a subjectkeyID) and an optional collection of
certificates and revocation/status information which may be used by the recipient.
IssuerSerial contains the encoded issuer name (RFC 2253) along with the serial number.

Object is an optional element that may occur one or more times. When present,
this element may contain any data. The Object element may include optional
MIME type, ID, and encoding attributes.

The MimeType attribute is an optional attribute which describes the data
within the Object. This is a string with values defined by [MIME].
For example, if the Object contains XML, the MimeType could be text/xml.
This attribute is purely advisory, no validation of the MimeType information is
required by this specification.

The Object's ID is commonly referenced from a Reference
in SignedInfo, or Manifest. This element is typically used for enveloping signatures where the
object being signed is to be included in the signature document. The digest is calculated
over the entire Object element including start and end tags.

Note, if the application wishes to exclude the <Object> tags from
the digest calculation the Reference must identify the actual data object
(easy for XML documents) or a transform must be used to remove the Object
tags (likely where the data object is non-XML). Exclusion of the object tags may be
desired for cases where one wants the signature to remain valid if the data object is
moved from inside a signature to outside the signature (or vice-versa), or where the
content of the Object is an encoding of an original binary document and it is
desired to extract and decode so as to sign the original bitwise representation.

This section describes the optional to implement Manifest and SignatureProperties
elements and describes the handling of XML Processing Instructions and Comments. With
respect to the elements Manifest and SignatureProperties this
section specifies syntax and little behavior -- it is left to the application. These
elements can appear anywhere the parent's content model permits; the Signature
content model only permits them within Object.

The Manifest element provides a list of References. The
difference from the list in SignedInfo is that it is application defined
which, if any, of the digests are actually checked against the objects referenced and what
to do if the object is inaccessible or the digest compare fails. If a Manifest
is pointed to from SignedInfo, the digest over the Manifest
itself will be checked by the core signature validation behavior. The digests within such
a Manifest are checked at application discretion. If a Manifest
is referenced from another Manifest, even the overall digest of this two
level deep Manifest might not be checked.

Additional information items concerning the generation of the signature(s) can be
placed in a SignatureProperty element (i.e., date/time stamp or the serial
number of cryptographic hardware used in signature generation.)

Note that PIs placed inside SignedInfo by an application will be signed
unless the CanonicalizationMethod algorithm discards them. (This is true for
any signed XML content.) All of the CanonicalizationMethods specified within
this specification retain PIs. When a PI is part of content that is signed (e.g., within SignedInfo
or referenced XML documents) any change to the PI will obviously result in a signature
failure.

Note that unless CanonicalizationMethod removes comments within SignedInfo
or any other referenced XML, they will be signed. Consequently, a change to the comment
will cause a signature failure. Similarly, the XML signature over any XML data will be
sensitive to comment changes unless a comment-ignoring canonicalization/transform method,
such as the Canonical XML [XML-canonicalization], is
specified.

This section identifies algorithms used with the XML digital signature standard.
Entries contain the identifier to be used in Signature elements, a reference
to the formal specification, and definitions, where applicable, for the representation of
keys and the results of cryptographic operations.

Algorithms are identified by URIs that appear as an attribute to the element that
identifies the algorithms' role (DigestMethod, Transform, SignatureMethod,
or CanonicalizationMethod). All algorithms used herein take parameters but in
many cases the parameters are implicit. For example, a SignatureMethod is
implicitly given two parameters: the keying info and the output of CanonicalizationMethod
(or SignedInfo directly if there is no CanonicalizationMethod).
Explicit additional parameters to an algorithm appear as content elements within the
algorithm role element. Such parameter elements have a descriptive element name, which is
frequently algorithm specific, and MUST be in the XML Signature namespace or an algorithm
specific namespace.

This specification defines a set of algorithms, their URIs, and requirements for
implementation. Requirements are specified over implementation, not over requirements for
signature use. Furthermore, the mechanism is extensible, alternative algorithms may be
used by signature applications.

Only one digest algorithm is defined herein. However, it is expected that one or more
additional strong digest algorithms will be developed in connection with the US Advanced
Encryption Standard effort. Use of MD5 [MD5] is NOT RECOMMENDED because recent advances in cryptography have
cast doubt on its strength.

The SHA-1 algorithm [SHA-1] takes no explicit parameters. An example of an SHA-1
DigestAlg element is:

<DigestMethod Algorithm="&dsig;sha1"/>

A SHA-1 digest is a 160-bit string. The content of the DigestValue element shall be the
base64 encoding of this bit string viewed as a 20-octet octet stream. Example, the
DigestValue element for the message digest:

MAC algorithms take two implicit parameters, their keying material determined from KeyInfo
and the byte stream output by CanonicalizationMethod or SignedInfo
directly if there is no CanonicalizationMethod. MACs and signature algorithms
are syntactically identical but a MAC implies a shared secret key.

The output of the HMAC algorithm is ultimately the output (possibly truncated) of the
chosen digest algorithm. This value shall be base64 encoded in the same straightforward
fashion as the output of the digest algorithms. Example: the SignatureValue element for
the HMAC-SHA1 digest

Signature algorithms take two implicit parameters, their keying material determined
from KeyInfo and the byte stream output by CanonicalizationMethod
or SignedInfo directly if there is no CanonicalizationMethod.
Signature and MAC algorithms are syntactically identical but a signature implies public
key cryptography.

Note: the schema and DTD declarations within this section are not yet
part of section 9: schemas.

The DSA algorithm [DSS] takes no explicit parameters. An example
of a DSA SignatureMethod element is:

<SignatureMethod Algorithm="&dsig;dsa"/>

The output of the DSA algorithm consists of a pair of integers usually referred by the
pair (r, s). The signature value consists of the base64 encoding of the concatenation of
two octet-streams that respectively result from the octet-encoding of the values r and s.
Integer to octet-stream conversion must be done according to the I2OSP operation defined
in the RFC 2437 [PKCS1]
specification with a k parameter equal to 20. For example, the SignatureValue element for
a DSA signature (r, s) with values specified in hexadecimal:

DSA key values have the following set of fields: P, Q, G and Y are mandatory when
appearing as a key value, J, seed and pgenCounter are optional but SHOULD be present. (The
seed and pgenCounter fields MUST appear together or be absent). All parameters are encoded
as base64 values.

The expression "RSA algorithm" as used in this specification refers to the
RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1].
The RSA algorithm takes no explicit parameters. An example of an RSA SignatureMethod
element is:

<SignatureMethod Algorithm="&dsig;rsa-sha1"/>

The output of the RSA algorithm is an octet string. The SignatureValue content for an
RSA signature shall be the base64 encoding of this octet string. Example: TBD

A Transform algorithm has three implicit parameters. The first is a byte
stream from the Reference or as the output of an earlier Transform.
The second and third are the optional MimeType and Charset
attributes that can be specified on the Transform element.

Application developers are strongly encouraged to support all transforms listed in this
section as RECOMMENDED unless the application environment has resource constraints that
would make such support impractical. The Working Group goal is to maximize application
interoperability on XML signatures, and the working group expects ubiquitous availability
of software to support these transforms that can be incorporated into applications without
extensive development.

The normative specification for base 64 and quoted-printable decoding transforms is [MIME]. Neither the base-64 nor the quoted-printable Transform
element has content. The input is decoded by the algorithms. This transform is useful if
an application needs to sign the raw data associated with the encoded content of an
element. Quoted-printable is provided, in addition to base-64, in keeping with the XML
support of a roughly human readable final format.

The XPath transform output is the result of applying an XPath
expression to an input string. The XPath expression appears in a parameter element named XPath.
The input string is equivalent to the result of dereferencing the URI attribute of the Reference
element containing the XPath transform, then, in sequence, applying all transforms that
appear before the XPath transform in the Reference element's Transforms.

The primary purpose of this transform is to omit information from the input document
that must be allowed to vary after the signature is affixed to the input document. It is
the responsibility of the XPath expression author to ensure that all information the
authentication of which is necessary has been included in the output such that
modification of the excluded information does not affect the secure interpretation of the
data in the application context. One simple example of this is the omission of an
enveloped signature's SignatureValue element.

6.6.3.1 Evaluation Context Initialization

The XPath transform establishes the following evaluation context for the XPath
expression given in the XPath parameter element:

A context node, initialized to null.

A context position, initialized to 0.

A context size, initialized to 0.

A library of functions equal to the function set defined in XPath plus the functions parse() and serialize() defined in this specification.

A set of variable bindings containing the variables $exprEncoding,
$exprBOM, and $input.

$exprEncoding: a string containing the character encoding of the XPath expression

$exprBOM: a string containing the byte order mark for the XPath expression; set to the
empty string if the document containing the XPath expression has no byte order mark.

$input: the string containing the input XML document, including the byte order mark, if
one exists. (Typically, $input is passed directly to parse(), but if $input does not
contain a well-formed XML document, XPath functions such as concat() can be used before
passing the result to parse()).

An empty set of namespace declarations. (Note: It is possible to
address a node by its qualified name, even though the evaluation context has not been
initialized with a declaration of the namespace. The XPath language provides the functions
namespace-uri() and local-name() for this purpose).

The XPath implementation is expected to convert all strings appearing in the XPath
expression to the same encoding used by the input string prior to making any comparisons.

The XPath specification defines a node-set to be unordered.
However, the specification also defines the notion of document order, and
it is clear that implementations must maintain knowledge of the document order in order to
correctly process the proximity position of a node. In XPath, a node's
position in the document order is given by the location of the first character of the
node's representative text in the document, except that an element's namespace nodes are
defined to be before its attribute nodes and the relative order of namespace nodes and
attribute nodes is application dependent. Within the XML-Signature application of XPath,
two namespace/attribute orderings are defined:

Lexicographic Order: the namespace and attribute axes are
lexicographically sorted on input, with namespace URI as primary key and local name as
secondary key. On serialization, the namespace nodes are placed before the attribute
nodes.

Exact Order: as with all other types of nodes, each namespace and
attribute node is associated with an integer P indicating the location of the first
character of its representative text, and the namespace and attribute axes are sorted by P.
On serialization, the namespace and attribute axes are merged using P as the key.

This function converts the Input string into a node-set. The function
throws an exception if it cannot provide the functionality corresponding to the LexOrder
setting or if the string does not contain a well-formed XML document (including byte order
mark if the document has one).

Because parse() uses an XML processor to read the input, linefeeds are normalized,
attribute values are normalized, CDATA sections are replaced by their content, and entity
references are recursively replaced by substitution text. In addition, any consecutive
characters are grouped into a single text node.

Although an XML processor reads the input XML document, validating and non-validating
XML processors only behave in the same way (e.g. with respect to attribute value
normalization and entity reference definition) until an external reference is encountered.
If the implementation uses a non-validating processor, and it encounters an external
reference in the input document, then the function should throw an exception to indicate
that the necessary algorithm is unavailable (The XPath transform cannot simply generate
incorrect output since many applications distinguish between an unverifiable signature
versus an invalid signature).

The node-set returned by this function has a context node of the root of the input XML
document, and the context position and context size are equal to 1. The function also
associates a document order position P with each node. For attribute and namespace
nodes, the value of P is dependent upon the LexOrder parameter. If
the LexOrder is false, then P is assigned using exact order as
defined in the previous section. If LexOrder
is true, then the value of P for namespace and attribute nodes is assigned based on
a lexicographic ordering of the namespace and attributes (as defined in the previous section). For a given element E with document
order position P, N namespace nodes and A attribute nodes, the successive namespace nodes
are assigned document order positions P+1 to P+N, and the successive attribute nodes are
assigned document order positions P+N+1 to P+N+A.

The function associates two strings with the root node: BOM and XMLDecl. The BOM
string contains the byte order mark or the empty string if there was no byte order mark.
The XMLDecl strings contain the complete, unaltered input text that the XML
processor absorbs while recognizing the 'XMLDecl' production rule.

The function associates a namespace-prefix string with each element, attribute and
namespace node to store the namespace prefix of namespace qualified nodes. The string is
empty unless the name of the node is namespace qualified.

This function converts a node-set into a string by generating the representative text
for each node in the node-set. The nodes of a node-set are processed in ascending order of
the nodes' P values (document order positions) as assigned by the parse() function.
The method of text generation is dependent on the node type and given in the following
list:

Root Node- The BOM (byte order mark) string then the XMLDecl string.
(Note that parse() does not preserve the document type declaration (the text that was
absorbed while matching the 'doctypedecl' production rule). This is because XPath provides
no access to the DTD or even node type for storing the DTD, so there is nothing with which
to associate a document order position.)

Element Nodes- An open angle bracket (<), the element name, any
namespace and attribute nodes in document order given by P, then a close angle
bracket (>), the descendant nodes of the element that are in the node-set in document
order, an open angle bracket, a forward slash (/), the element name, and a close angle
bracket. The element name is either (1) the local name if the namespace prefix string is
empty or (2) the namespace prefix and a colon followed by the local name of the element.

Namespace and Attribute Nodes- a space, the namespace prefix and a
colon if the namespace prefix string for the node is non-empty, the local name, an equals
sign, an open double quote, the modified string value, the attribute value, and a close
double quote. The string value of the node is modified by replacing all ampersands (&)
with &amp;, all double quote characters with &quot;, and
all illegal characters for the output character encoding with hexadecimal character
references (e.g. &#x0D;).

Text Nodes- the string value, except all ampersands are replaced by &amp;,
all open angle brackets (<) are replaced by &lt;, and all illegal
characters for the output character encoding with hexadecimal character references (e.g. &#x0D;).

Processing Instruction Nodes- an open angle bracket, a question mark,
the expanded name of the node, a space, the string value, the question mark, and a close
angle bracket.

Comment Nodes- the open comment sequence (<!--), the string value of
the node, and the close comment sequence (-->).

The result of the XPath expression is a string, boolean, number, or node-set. If the
result of the XPath expression is a string, then the string is the output of the XPath
transform. If the result is a boolean or number, then the XPath transform output is
computed by calling the XPath string() function on the boolean or number. If the result of
the XPath expression is a node-set, then the XPath transform output is the string result
of calling serialize() on the node-set.

For example, consider creating an enveloped signature S1 (a Signature
element with an id attribute equal to "S1"). The signature S1 is
enveloped because its Reference URI indicates some ancestor element of S1.
Since the DigestValue in the Reference is calculated before S1's
SignatureValue, the SignatureValue must be omitted from the DigestValue
calculation. This can be done with an XPath transform containing the following XPath
expression in its XPath parameter element:

serialize(parse($input, "true")/descendant-or-self::node()[
not(self::SignatureValue and
parent::Signature[@id="S1"]) and
not(self::KeyInfo and parent::Signature[@id="S1"])
and
not(self::DigestValue and ancestor::*[3 and
@id="S1"])]

The parse() call creates a node-set from the $input using lexicographic order for the
namespace and attribute order. The '/descendant-or-self::node()' means that all nodes in
the entire parse tree starting at the root node are candidates for the result node-set.
For each node candidate, the node is included in the resultant node-set if and only if the
node test (the boolean expression in the square brackets) evaluates to "true"
for that node. The node test returns true for all nodes except the SignatureValue
and KeyInfo child elements and the DigestValue descendants of Signature
S1. Thus, serialize() returns a string containing the entire $input except for omitting
the parts of S1 that must change during core processing, so these changes will not
invalidate a DigestValue computed over the serialize() result.

Note that this expression works even if the XPath transform is implemented with a
non-validating processor because S1 is identified by comparison to the value of an
attribute named 'id' rather than by using the XPath id() function. Although the id()
function is useful when the 'id' attribute is not named 'id', the XPath expression author
will know the 'id' attribute's name when writing the expression.

It is RECOMMENDED that the XPath be constructed such that the result of this operation
is a well-formed XML document. This should be the case if root element of the input
resource is included by the XPath (even if a number of its descendant nodes are omitted by
the XPath expression). It is also RECOMMENDED that nodes should not be omitted from the
input if they affect the interpretation of the output nodes in the application context.
The XPath expression author is responsible for this since the XPath expression author
knows the application context.

The Transform element contains a single parameter child element called XSLT,
whose content MUST conform to the XSL Transforms [XSLT] language syntax. The
processing rules for the XSLT transform are stated in the XSLT specification [XSLT].

Digital signatures only work if the verification calculations are performed on exactly
the same bits as the signing calculations. If the surface representation of the signed
data can change between signing and verification, then some way to standardize the
changeable aspect must be used before signing and verification. For example, even for
simple ASCII text there are at least three widely used line ending sequences. If it is
possible for signed text to be modified from one line ending convention to another between
the time of signing and signature verification, then the line endings need to be
canonicalized to a standard form before signing and verification or the signatures will
break.

XML is subject to surface representation changes and to processing which discards some
surface information. For this reason, XML digital signatures have a provision for
indicating canonicalization methods in the signature so that a verifier can use the same
canonicalization as the signer.

Throughout this specification we distinguish between the canonicalization of a Signature
data object and other signed XML data objects. It is possible for an isolated XML document
to be treated as if it were binary data so that no changes can occur. In that case, the
digest of the document will not change and it need not be canonicalized if it is signed
and verified as such. However, XML that is read and processed using standard XML parsing
and processing techniques is frequently changed such that some of its surface
representation information is lost or modified. In particular, this will occur in many
cases for the Signature and enclosed SignedInfo elements since
they, and possibly an encompassing XML document, will be processed as XML.

Similarly, these considerations apply to Manifest, Object,
and SignatureProperties elements if those elements have been digested, their DigestValue
is to be checked, and they are being processed as XML.

The kinds of changes in XML that may need to be canonicalized can be divided into three
categories. There are those related to the basic [XML], as described in 7.1 below. There
are those related to [DOM], [SAX], or
similar processing as described in 7.2 below. And, third, there is the possibility of
character set conversion, such as between UTF-8 and UTF-16, both of which all XML
standards compliant processors are required to support. Any canonicalization algorithm
should yield output in a specific fixed character set. For both the minimal
canonicalization defined in this specification and the W3C Canonical XML [XML-c14n],
that character set is UTF-8.

XML 1.0 [XML]
defines an interface where a conformant application reading XML is given certain
information from that XML and not other information. In particular,

line endings are normalized to the single character #xA by dropping #xD characters if
they are immediately followed by a #xA and replacing them with #xA in all other cases,

missing attributes declared to have default values are provided to the application as if
present with the default value,

character references are replaced with the corresponding character,

entity references are replaced with the corresponding declared entity,

attribute values are normalized by

replacing character and entity references as above,

replacing occurrences of #x9, #xA, and #xD with #x20 (space) except that the sequence
#xD#xA is replaced by a single space, and

if the attribute is not declared to be CDATA, stripping all leading and trailing spaces
and replacing all interior runs of spaces with a single space, and

for elements declared to have element content, eliminate white space that appears within
their content but not within the content of any enclosed element.

Note that items (2), (4), (5C), and (6) depend on specific schema, DTD, or similar
declarations. In the general case, such declarations will not be available to or used by
the signature verifier. Thus, to interoperate between different XML implementations, the
following syntax contraints MUST be observed when generating any signed material to be
processed as XML, including the SignedInfo element:

attributes having default values be explicitly present,

all entity references (except "amp", "lt", "gt",
"apos", and "quot" which are pre-defined) be expanded,

attribute value white space be normalized, and

insignificant white space not be generated within elements having element content.

In addition to the canonicalization and syntax constraints discussed above, many XML
applications use the Document Object Model [DOM]
or The Simple API for XML [SAX].
DOM maps XML into a tree structure of nodes and typically assumes it will be used on an
entire document with subsequent processing being done on this tree. SAX converts XML into
a series of events such as a start tag, content, etc. In either case, many surface
characteristics such as the ordering of attributes and insignificant white space within
start/end tags is lost. In addition, namespace declarations are mapped over the nodes to
which they apply, losing the namespace prefixes in the source text and, in most cases,
losing where namespace declarations appeared in the original instance.

If an XML Signature is to be produced or verified on a system using the DOM or SAX
processing, a canonical method is needed to serialize the relevant part of a DOM tree or
sequence of SAX events. XML canonicalization specifications, such as [XML-c14n],
are based only on information which is preserved by DOM and SAX. For an XML Signature to
be verifiable by an implementation using DOM or SAX, not only must the syntax constraints
given in section
7.1 be followed but an appropriate XML canonicalization MUST be specified so that the
verifier can re-serialize DOM/SAX mediated input into the same byte sequence that was
signed.

A requirement of this specification is to permit signatures to "apply to a
part or totality of a XML document." (See section 3.1.3 of [XML-Signature-RD])
The Transforms mechanism meets this requirement by permitting one to sign
data derived from processing the content of the identified resource. For instance,
applications that wish to sign a form, but permit users to enter limited field data
without invalidating a previous signature on the form might use XPath [XPath]
to exclude those portions the user needs to change. Transforms may be
arbitrarily specified and may include canonicalization instructions or even XSLT
transformations. Of course, signatures over such a derived document do not secure any
information discarded by the Transforms.

Furthermore, core
validation behavior does not confirm that the signed data was obtained by applying
each step of the indicated transforms. (Though it does check that the digest of the
resulting content matches that specified in the signature.) For example, some
application may be satisfied with verifying an XML signature over a cached copy of already
transformed data. Other application might require that content be freshly dereferenced and
transformed.

If signing is intended to convey the judgment or consent of an automated mechanism or
person, then it is normally necessary to secure as exactly as practical the information
that was presented to that mechanism or person. Note that this can be accomplished by
literally signing what was presented, such as the screen images shown a user. However,
this may result in data which is difficult for subsequent software to manipulate. Instead,
one can sign the data along with whatever filters, style sheets, client profile or other
information that affects its presentation.

Also note that the use of Canonical XML [XML-C14N]
ensures that all internal entities and XML namespaces are expanded within the content
being signed. All entities are replaced with their definitions and the canonical form
explicitly represents the namespace that an element would otherwise inherit. Those
application that do not canonicalize XML content (especially the SignedInfo
element) SHOULD NOT use internal entities and SHOULD represent the name space explicitly
within the content being signed since they can not rely upon canonicalization to do this
for them.

This standard specifies public key signatures and keyed hash authentication codes.
These have substantially different security models. Furthermore, it permits user specified
algorithms which may have other models.

With public key signatures, any number of parties can hold the public key and verify
signatures while only the parties with the private key can create signatures. The number
of holders of the private key should be minimized and preferably be one. Confidence by
verifiers in the public key they are using and its binding to the entity or capabilities
represented by the corresponding private key is an important issue, usually addressed by
certificate or online authority systems.

Keyed hash authentication codes, based on secret keys, are typically much more
efficient in terms of the computational effort required but have the characteristic that
all verifiers need to have possession of the same key as the signer. Thus any verifier can
forge signatures.

This standard permits user provided signature algorithms and keying information
designators. Such user provided algorithms may have different security models. For
example, methods involving biometrics usually depend on a physical characteristic of the
authorized user that can not be changed the way public or secret keys can be and may have
other security model differences.

The strength of a particular signature depends on all links in the security chain. This
includes the signature and digest algorithms used, the strength of the key generation [RANDOM]
and the size of the key, the security of key and certificate authentication and
distribution mechanisms, certificate chain validation policy, protection of cryptographic
processing from hostile observation and tampering, etc.

Care must be exercised by validaters in executing the various algorithms that may be
specified in an XML signature and in the processing of any "executable content"
that might be provided to such algorithms as parameters, such as XSLT transforms. The
algorithms specified in this document will usually be implemented via a trusted library
but even there perverse parameters might cause unacceptable processing or memory demand.
Even more care may be warranted with application defined algorithms.

The security of an overall system will also depend on the security and integrity of its
operating procedures, its personnel, and on the administrative enforcement of those
procedures. All the factors listed in this section are important to the overall security
of a system; however, most are beyond the scope of this specification.

The actual binary/octet data being operated on (transformed, digested, or signed) by an
application -- frequently an HTTP entity [HTTP]. Note that the proper noun Object designates a
specific XML element. Occasionally we refer to a data object as a document or as
a resource's content. The term element
content is used to describe the data between XML start and end tags [XML]. The term XML document is used to describe data objects
which conform to the XML specification [XML].

"A resource can be anything that has identity. Familiar examples include an
electronic document, an image, a service (e.g., 'today's weather report for Los Angeles'),
and a collection of other resources.... The resource is the conceptual mapping to an
entity or set of entities, not necessarily the entity which corresponds to that mapping at
any particular instance in time. Thus, a resource can remain constant even when its
content---the entities to which it currently corresponds---changes over time, provided
that the conceptual mapping is not changed in the process." [URI]
In order to avoid a collision of the term entity within the URI and XML
specifications, we use the term data object, content or document
to refer to the actual bits being operated upon.

The signature is over content external to the Signature element, and can be
identified via a URI, IDREF, or transform. Consequently, the
signature is "detached" from the content it signs. This definition typically
applies to separate data objects, but it also includes the instance where the Signature
and data object reside within the same XML document but are sibling elements.

The signature is over the XML content that contains the signature as an element. The
content provides the root XML document element. Obviously, enveloped signatures must take
care not to include their own value in the calculation of the SignatureValue.

The application determines that the semantics associated with a signature are valid. For
example, an application may validate the time stamps or the integrity of the signer key --
though this behavior is external to this core
specification.